In recent years, alkaline storage batteries are in wide use ranging over power sources of various portable equipment to large size batteries for electric vehicles. In these alkaline batteries, nickel hydroxide electrode is generally employed as the positive electrode.
The methods of manufacturing nickel hydroxide electrodes can be broadly divided into two types, namely, sintered type and paste type. Sintered electrodes are made by immersing in an alkaline solution after immersing in a nickel nitrate solution or nickel sulfate solution and suffer the problem of complication of the manufacturing process and a low capacity density. On the other hand, paste type nickel electrodes are made by filling an active material primarily composed of nickel hydroxide onto a high-porosity foamed or fibrous substrate; their manufacturing process is simple and it is possible to achieve a high capacity.
As the nickel hydroxide itself has a low electric conductivity, rate of utilization of the positive active material tends to be low when filling it alone on the substrate because transfer of electrons cannot be smoothly performed. In order to solve this problem, a method is widely employed in which a cobalt compound is added to the positive active material paste. This is based on the understanding that the cobalt compound is easily oxidized during the initial charge to become high electric-conductivity cobalt oxyhydroxide thus functioning as electrically conducting networks among nickel hydroxide particles and between the current collector and nickel hydroxide particles. However, as cobalt is rare, it is necessary to form enough electrically conducting networks by addition of a smaller quantity of cobalt compound in order to provide a lower-cost battery as well as to achieve a further higher capacity.
In order to solve these problems, many proposals have been made as to the quantity and conditions of adding cobalt compounds such as metallic cobalt, cobalt oxide, cobalt hydroxide, etc. More recently, proposals have been made to cover the surface of nickel hydroxide particles with cobalt hydroxide to obtain an enhanced effect of addition of cobalt hydroxide of a smaller quantity.
For example, Japanese Laid-Open Patent Application No. Hei 9-45323 proposes a process of obtaining an active material for alkaline storage batteries by adding nickel hydroxide powder into an aqueous solution of cobalt compound followed by neutralizing in an aqueous solution adjusted to pH 10-12, then suspending in an alkaline aqueous solution and electrochemically oxidizing it. However, the cobalt oxyhydroxide obtained by electrochemical oxidation of xcex2-type cobalt hydroxide has a specific electric conductivity on the order of 10xe2x88x925 S/cm, which is not very high. This process also suffers the problem of including a step of electrochemical oxidation of powders which tends to be complicated.
Also, in Japanese Laid-Open Patent Application No. Hei 8-148146, for example, it is proposed to improve the rate of utilization of the positive active material by producing higher-order cobalt oxides having a high electric conductivity on the surface of nickel hydroxide by heat treatment of nickel hydroxide of which the surface has been covered with cobalt hydroxide in an environment of coexisting oxygen and alkali metal hydroxides. However, in this method, tricobalt tetraoxide tends to be produced as a by-product while cobalt hydroxide is being oxidized, suggesting that not all the cobalt compound is effectively functioning thus not exhibiting full effect.
Also, as the reaction is one which takes place at the interface among vapor phase, solid phase, and liquid phase, the method has a drawback of suffering considerably high non-uniformity of reaction and of complication of the process of producing a high electric-conductivity cobalt compound on the surface of nickel hydroxide powder.
Further, though Japanese Laid-Open Patent Application No. Hei 10-125315 discloses an active material primarily composed of nickel hydroxide the surface of which is covered with a cobalt compound, it has been difficult to uniformly and stably produce cobalt oxyhydroxide having an especially high electric conductivity on nickel hydroxide.
The present invention addresses the previously existing problems as described above. It is an object of the invention to eliminate the problems encountered in oxidizing cobalt hydroxide and to produce cobalt oxyhydroxide with a high electric conductivity on the surface of nickel hydroxide with uniformity and stability while simplifying the process, thus improving the rate of utilization of the positive active material and discharge characteristic of alkaline storage batteries.
In accomplishing the above object, the present invention first produces xcex1-type cobalt hydroxide on the surface of nickel hydroxide of the positive active material of an alkaline storage battery, followed by oxidizing treatment of it with an oxidizing agent such as sodium hypochlorite or potassium permanganate thereby producing cobalt oxyhydroxide with a high electric conductivity on the surface of nickel hydroxide with simplicity, uniformity and stability.
The positive active material for alkaline storage batteries as obtained above is subsqeuntly filled onto foamed metal and the like to obtain an electrode for alkaline storage batteries. The present invention also intends to improve the rate of utilization of the positive active material and discharge characteristic by making its specific electric conductivity to within the range 1 to 10xe2x88x924 S/cm.
The present invention utilizes a positive active material obtained by dispersing nickel hydroxide in an aqueous solution of cobalt (II) such as cobalt nitrate or cobalt sulfate, neutralizing it with a hydroxide of an alkali metal to render xcex1-type cobalt hydroxide adhere on the surface of nickel hydroxide, followed by treating with an oxidizing agent to produce cobalt oxyhydroxide with a high electric conductivity on the surface of the active material. By forming a high electric conductivity cobalt oxyhydroxide on the surface of the active material, the low rate of utilization of the positive active material attributable to the property of a low electric-conductivity of nickel hydroxide can be significantly improved.
Here, it is conceivable that the higher the electronic conductivity of the cobalt hydroxide on the surface is, the more improved are the rate of utilization of the positive active material and discharge characteristic. According to the present invention, the specific electric conductivity can be made to such a high value as 1 to 10xe2x88x924 S/cm.
Also, as the present invention uses xcex1-type cobalt hydroxide as the starting material, oxidizing treatment is simple, namely, xcex1-type cobalt hydroxide can be rapidly converted into a higher-order cobalt oxide with a high electric conductivity by simply adding an oxidizing agent such as aqueous solution of sodium hypochlorite, aqueous solution of hydrogen peroxide, aqueous solution of potassium permanganate, and the like. This reaction being a reaction at a solid-liquid interface, the oxidizing treatment can be performed relatively uniformly. Furthermore, this process can be performed in a batch immediately after the above-described process of coating the surface of nickel hydroxide particles by xcex1-type cobalt hydroxide.
In doing this, it is good to keep the pH of the reaction mixture to be used in rendering xcex1-type cobalt hydroxide adhere on the surface of nickel hydroxide in the range 8 to 10. When the pH of the reaction mixture is too low, it is not possible to produce enough quantity of xcex1-type cobalt hydroxide on the nickel hydroxide particle surface. Also, when the pH is higher than 10, greenish-blue xcex1-type cobalt hydroxide rapidly changes to white-peach colored xcex2 type and the subsequent oxidizing treatment with an aqueous solution of sodium hypochlorite is made difficult. Furthermore, when stirring efficiency during subsequent neutralization is poor, the pH locally becomes higher than 10 making it difficult to uniformly produce xcex1-type cobalt hydroxide on the surface of nickel hydroxide particles.
Also, in the above active material of the present invention, the quantity of cobalt oxyhydroxide covering nickel hydroxide surface is chosen to be 0.5 to 10% by weight of nickel hydroxide. When the quantity of xcex1-type cobalt hydroxide covering nickel hydroxide surface is too small, it is not possible to provide a high enough electric conductivity on the surface thus not exhibiting its effect. Conversely, when the quantity is too large, a large quantity of expensive cobalt compound will be consumed thus making the cost high while causing an undesirable decrease in the filling density of the positive active material. The adequate quantity of xcex1-type cobalt hydroxide for covering is 0.5 to 10% by weight of nickel hydroxide, preferably 2 to 5%.
As has been described above, with the present invention, a positive active material having a high rate of utilization and a superior discharge characteristic can be produced more easily by the addition of a smaller quantity of the cobalt compound.